In order for a body to function correctly cells must work together. There are usually many different types of cells in a multi-cellular organism; each type of cell has its own specific function. Groups of similar cells make tissues; groups of similar tissues make organs. Groups of organs working together make organ systems. The following sections describe many of the organ systems inside complex animals like humans. There are other systems of the body that are not covered here. This is only covering the basics. Other systems in our bodies that aren’t covered here include: reproductive, lymphatic (disease fighting), urinary, and integumentary (skin) systems.